1. Field
The disclosed concept pertains generally to nuclear reactors and, more particularly, to nuclear assemblies that employ guide thimbles with enhanced stiffness in the dashpot region.
2. Related Art
In most water cooled nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. In pressurized water nuclear reactors (PWR), these fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles are on opposite ends of the fuel assembly and are secure to the ends of the thimble tubes that extend above and below the ends of the fuel rods.
When the control rods scram, they freefall and can impact the bottom nozzle at a high velocity, potentially causing damage to components of the nuclear reactor. In a standard fuel assembly design, approximately two feet before full insertion of the control rods into the fuel assembly, the tips of the control rods enter a small portion of the thimble tube called the dashpot. The diameter of the dashpot is approximately one millimeter larger than the control rods. Because the control rods are moving very fast at this point in the scram, there is a large volume of water which has to be accelerated up past the falling rods to make room for them in the dashpot. This process causes the control rods to decelerate rapidly, thus lessening the impact velocity of the falling control rod.
Incomplete rod insertion (IRI) events are problematic in nuclear reactors. An IRI event occurs when the control rod cannot be completely inserted through the thimble tube. One of the primary causes of an IRI event is a distortion in the fuel assembly and the thimble tube. The distortion is most critical in the dashpot area of the thimble tube where the clearance between the control rod and the inner surface of the thimble tube is minimal. A lack of stiffness in the dashpot region of the thimble tube can cause the thimble tube to be susceptible to distortion and increase the chance of an IRI event occurring.
There is a need to improve the stiffness of the dashpot region of thimble tubes. However, there is also a need to minimize the number of parts and assembly steps of fuel assemblies.